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Kinematics - Acceleration in Two Dimensions. Stuck.

  • Thread starter mattstjean
  • Start date
  • #1
14
0
Hi, I am also having trouble with the hockey puck question.

A hockey puck rebounds from a board as shown in my diagram. The puck is in contact with the board for 2.5 ms. Determine avg acceleration of the puck over the interval.

Vi = 26 m/s Vf = 21 m/s

I tried the cosine law but I keep getting 44 m/s and not 18. I don't understand how you guys got 18 m/s. I've plugged it in at least 100 times as
[itex]
v_t = \sqrt{v_1^2 + v_2^2 - 2(v_1)(v_2)cos136}
= \sqrt{26^2 + 21^2 - 2(26)(21)cos136}
=44
= [/itex]

Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:
[itex]
V_x = V_B sin \theta + (-V_A cos \beta )
= 21 sin(22) - 26 cos(22)
=-16
[/itex]

and

[itex]
V_y = V_B cos \theta + (-V_A sin \beta )
= 21 (cos22) + 26(sin22)
= 29
[/itex]

I then tried to figure out
[itex]
\Delta V ^2= \Delta V_x ^2 + \Delta V_y^2
= sqrt{16^2 + 29^2}
= 33
[/itex]

Using that I tried to get the average acceleration by:

[itex]
A_av = \Delta V / \Delta T

A_av = 33 / 2.5x10^-3
A_av = 13.2x10^3
[/itex]
and to find the angle I tried to do :

[itex]
\phi = tan^-1 = 16/29
\phi = 29degrees
[/itex]

However, the answer in my book says that the average acceleration is [itex] 7.3x10^3 [7.5degrees North of West][/itex]



Any help would be amazingly appreciated. Thanks.
 

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Answers and Replies

  • #2
Doc Al
Mentor
44,940
1,201
Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:
[itex]
V_x = V_B sin \theta + (-V_A cos \beta )
= 21 sin(22) - 26 cos(22)
=-16
[/itex]
You have a mix of sine and cosine. Only one is correct.
and

[itex]
V_y = V_B cos \theta + (-V_A sin \beta )
= 21 (cos22) + 26(sin22)
= 29
[/itex]
Again, a mix of sine and cosine.

Redo this.
 
  • #3
14
0
You have a mix of sine and cosine. Only one is correct.

Again, a mix of sine and cosine.

Redo this.
I don't know how to redo it. In my text book it used them both together in the y and x component vector subtraction. I took the equations right out of my text, Nelson Physics 12.
 
  • #4
Doc Al
Mentor
44,940
1,201
I don't know how to redo it. In my text book it used them both together in the y and x component vector subtraction. I took the equations right out of my text, Nelson Physics 12.
I'm not sure what equations you are talking about.

Do this: What's the x-component of Vi? The x-component of Vf?
 
  • #5
1
0
I have the same book and I am stuck on the example on right before the quesion box you asked about. can you please explain the lawn mower example in pg 28. I AM VERY CONFUSED
 

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